1. Field of the Invention
The present invention relates generally to the fields of molecular biology and therapy for pleural diseases. More particularly, the present invention concerns single chain urokinase plasminogen activator (scuPA) in pleural disease.
2. Description of Related Art
The pathogenesis of pleural loculation recapitulates that of wound healing, with a progression of extravascular fibrin deposition and remodeling of transitional fibrin leads to fibrotic repair and scar formation (Dvorak, 1986). The injured pleural space is an inhibitor-rich environment, which limits expression of endogenous PA activity. Pleural fluid fibrinolytic activity is generally depressed in exudative pleural effusions and in this respect recapitulates the response observed in alveolar lining fluids in ARDS (Idell, 1994). For example, there is almost always no detectable fibrinolytic activity in pleural fluids from patients with parapneumonic effusions and empyema and procoagulant activity is concurrently augmented (Idell et al., 1991). These conditions favor the deposition and maintenance of intrapleural fibrin in the setting of high-grade local inflammation. While the pleural fluids contain both tPA and uPA, most of the immunoreactive PA is bound and irreversibly inhibited by PAI, mainly PAI-1 (Idell et al., 1991). Only small amounts of immunoreactive free uPA, but not tPA are detectable and uPA is present in amounts insufficient to overcome inhibition by PAI-1 and antiplasmins that further damp the expression of fibrinolytic activity in these fluids. Fibrinolytic activity is likewise undetectable in pleural effusions that form after TCN-induced pleuritis in rabbits attributable to the identical expression of PAI-1 and antiplasmins (Strange et al., 1995). These circumstances likely limit the fibrinolytic activity of the currently used fibrinolysins.
A phase of fibrinous adhesion formation initiates intrapleural loculation and fibrosis (Sahn, 1988; Light, 2001). Morphologic evidence from TCN-induced pleural injury in rabbits directly links transitional extravascular fibrin deposition to the pathogenesis of intrapleural loculation after acute pleural injury (Idell et al., 1998; Strange et al., 1995). Intrapleural loculation can occur early in the course of complicated parapneumonic effusions or frank empyemas and can adversely affect clinical outcome (Sahn, 1988; Light, 2001). It has long been appreciated that an initial fibrinopurulent phase occurs as part of the inflammatory response. During this phase, intrapleural fibrin forms and can bridge the visceral and parietal pleural surfaces (Sahn, 1988; Light, 2001; Sahn, 1998a). If the inflammatory process persists, the fibrinous exudate undergoes organization, with fibroblast invasion and collagen deposition (Light, 2001). Clinically, formation of a “pleural peel” and extensive intrapleural organization can encase the lung and impair its function. A similar process of intrapleural fibrin deposition and organization can occasionally occur in association with hemothoraceses, supporting the concept that intrapleural fibrin formation is integral to subsequent loculation and scarring in the pleural compartment (Light, 2001).
Intrapleural fibrin deposition can be targeted for therapeutic benefit. Tillett and Sherry originally used relatively crude preparations of streptokinase to degrade pleural loculations (Tillett and Sherry, 1949). This fibrinolytic strategy has been refined and remains a viable clinical option for treatment of loculated parapneumonic effusions (Sahm, 1988; Light, 2001; Sahn, 1998b; Light et al., 1998; Colice et al., 2000). However, the use of intrapleural fibrinolysins can be logistically difficult and expensive in that multiple treatments are often required and hospital stay is often extended, providing the rationale to investigate whether alternative agents could be effective and safe for intrapleural application.
Previous studies have found that administration of intrapleural heparin or the no-longer commercially available low molecular weight (LMW) uPA; Abbokinase (Abbott Laboratories, North Chicago, Ill.) could attenuate intrapleural adhesion formation induced by intrapleural administration of TCN in rabbits. Multiple doses of heparin or intrapleural LMW uPA; every 12 hr over 72 hr were required to achieve partial protection against formation of intrapleural adhesions in this study. However, the protection provided by LMW uPA was found to be incomplete.
There is ongoing debate about the place of currently available fibrinolytic interventions for intrapleural loculation. In a recent systemic review of all available randomized controlled trials, the pooled data showed that small benefits of fibrinolytic intervention could be anticipated in terms of reduction of hospital stay, time to defervescence, radiographic improvement and ultimate need for surgical intervention (Cameron, 2000). Based on this analysis, it was concluded that there was insufficient rate evidence to support the routine use of fibrinolysins in this context. In another recent retrospective analysis, streptokinase was found to increase the volume of pleural fluid drained from patients with organized empyemas, but there was no significant impact on hospital stay, defervescence, need for surgical intervention or mortality (de Souza et al., 2000). These observations provide a clinically-based rationale for the investigation of new interventional fibrinolysins for intrapleural use.
When bound to its receptor; uPAR (either on the cell surface or in soluble form), scuPA expresses enhanced and sustained fibrinolytic activity that is also relatively resistant to plasminogen activator inhibitors (PAI) (Higazi et al, 1996; Higazi, 1995; Manchanda, 1995). These properties suggest that use of this agent could be of advantage in organizing pleuritis, where the local concentrations of PAI are very high (Idell et al., 1991). The inventors have previously shown that pleural mesothelial cells and fibroblasts express uPAR and that uPAR is upregulated in these cells by cytokines expressed in pleural injury (Shetty and Idell, 1998; Shetty et al., 1996).
Based on the above, there is a need for methods and compositions that allow for the prevention of pleural adhesion formation.